1. Field of the Invention
This invention relates to a variable magnification copying apparatus using an electrophotographic photosensitive medium.
2. Description of the Prior Art
In a variable magnification copying apparatus, the imaging magnification of an original image is changed by changing the ratio of the length of the forward light path of a lens to the length of the rearward light path of the lens. (For example, U.S. Pat. Nos. 3,542,467; 3,614,222 and 4,116,561).
When the imaging magnification is changed, the density of a light beam on a photosensitive medium varies even if a light beam of the same density is incident on the lens. In other words, the exposure amount of the photosensitive medium varies about the imaging magnification. To meet such inconvenience, there are U.S. Pat. No. 3,917,393, U.S. Application Ser. No. 138,981 now U.S. Pat. No. 4,264,198 (corresponding DT-OS No. 2839240), etc. The former discloses a technique of varying the incidence angle of the light beam with respect to a slit of fixed width in accordance with a selected magnification. However, the rate of variation in exposure amount corresponding to the variation in magnification is not simple, and such slit cannot accomplish sufficient correction of the quantity of light. In the case of the latter, the iris diaphragm of the lens is adjusted with a change of the magnification. However, this iris diaphragm is also used to adjust the image density as the operator likes. Accordingly, a complicated mechanism becomes necessary which prevents the diaphragm adjusting operation accompanying a magnification change from affecting a density adjusting dial. Also, by a mechanical diaphragm, it is considerably difficult to maintain the exposure amount of the photosensitive medium constant irrespective of a magnification change.